4 files changed, 53 insertions, 47 deletions

diff --git a/crates/hir_ty/src/infer/unify.rs b/crates/hir_ty/src/infer/unify.rs
index 3732d8ebd..c3d9e81c3 100644
--- a/crates/hir_ty/src/infer/unify.rs
+++ b/crates/hir_ty/src/infer/unify.rs
@@ -118,21 +118,10 @@ impl<'a, 'b> Canonicalizer<'a, 'b> {
 
 impl<T: HasInterner<Interner = Interner>> Canonicalized<T> {
     pub(super) fn decanonicalize_ty(&self, ty: Ty) -> Ty {
-        ty.fold_binders(
-            &mut |ty, binders| {
-                if let TyKind::BoundVar(bound) = ty.kind(&Interner) {
-                    if bound.debruijn >= binders {
-                        let (v, k) = self.free_vars[bound.index];
-                        TyKind::InferenceVar(v, k).intern(&Interner)
-                    } else {
-                        ty
-                    }
-                } else {
-                    ty
-                }
-            },
-            DebruijnIndex::INNERMOST,
-        )
+        crate::fold_free_vars(ty, |bound, _binders| {
+            let (v, k) = self.free_vars[bound.index];
+            TyKind::InferenceVar(v, k).intern(&Interner)
+        })
     }
 
     pub(super) fn apply_solution(
diff --git a/crates/hir_ty/src/lib.rs b/crates/hir_ty/src/lib.rs
index 7d4793c2f..60680ec86 100644
--- a/crates/hir_ty/src/lib.rs
+++ b/crates/hir_ty/src/lib.rs
@@ -302,3 +302,29 @@ pub fn dummy_usize_const() -> Const {
     }
     .intern(&Interner)
 }
+
+pub(crate) fn fold_free_vars<T: HasInterner<Interner = Interner> + Fold<Interner>>(
+    t: T,
+    f: impl FnMut(BoundVar, DebruijnIndex) -> Ty,
+) -> T::Result {
+    use chalk_ir::{fold::Folder, Fallible};
+    struct FreeVarFolder<F>(F);
+    impl<'i, F: FnMut(BoundVar, DebruijnIndex) -> Ty + 'i> Folder<'i, Interner> for FreeVarFolder<F> {
+        fn as_dyn(&mut self) -> &mut dyn Folder<'i, Interner> {
+            self
+        }
+
+        fn interner(&self) -> &'i Interner {
+            &Interner
+        }
+
+        fn fold_free_var_ty(
+            &mut self,
+            bound_var: BoundVar,
+            outer_binder: DebruijnIndex,
+        ) -> Fallible<Ty> {
+            Ok(self.0(bound_var, outer_binder))
+        }
+    }
+    t.fold_with(&mut FreeVarFolder(f), DebruijnIndex::INNERMOST).expect("fold failed unexpectedly")
+}
diff --git a/crates/hir_ty/src/lower.rs b/crates/hir_ty/src/lower.rs
index b45e811fa..109157a5e 100644
--- a/crates/hir_ty/src/lower.rs
+++ b/crates/hir_ty/src/lower.rs
@@ -1016,22 +1016,16 @@ pub(crate) fn generic_defaults_query(
                 p.default.as_ref().map_or(TyKind::Error.intern(&Interner), |t| ctx.lower_ty(t));
 
             // Each default can only refer to previous parameters.
-            ty = ty.fold_binders(
-                &mut |ty, binders| match ty.kind(&Interner) {
-                    TyKind::BoundVar(BoundVar { debruijn, index }) if *debruijn == binders => {
-                        if *index >= idx {
-                            // type variable default referring to parameter coming
-                            // after it. This is forbidden (FIXME: report
-                            // diagnostic)
-                            TyKind::Error.intern(&Interner)
-                        } else {
-                            ty
-                        }
-                    }
-                    _ => ty,
-                },
-                DebruijnIndex::INNERMOST,
-            );
+            ty = crate::fold_free_vars(ty, |bound, binders| {
+                if bound.index >= idx && bound.debruijn == DebruijnIndex::INNERMOST {
+                    // type variable default referring to parameter coming
+                    // after it. This is forbidden (FIXME: report
+                    // diagnostic)
+                    TyKind::Error.intern(&Interner)
+                } else {
+                    bound.shifted_in_from(binders).to_ty(&Interner)
+                }
+            });
 
             crate::make_only_type_binders(idx, ty)
         })
diff --git a/crates/hir_ty/src/method_resolution.rs b/crates/hir_ty/src/method_resolution.rs
index 7e09a1539..766880b14 100644
--- a/crates/hir_ty/src/method_resolution.rs
+++ b/crates/hir_ty/src/method_resolution.rs
@@ -6,7 +6,11 @@ use std::{iter, sync::Arc};
 
 use arrayvec::ArrayVec;
 use base_db::CrateId;
-use chalk_ir::{cast::Cast, Mutability, UniverseIndex};
+use chalk_ir::{
+    cast::Cast,
+    fold::{Fold, Folder},
+    Fallible, Mutability, UniverseIndex,
+};
 use hir_def::{
     lang_item::LangItemTarget, nameres::DefMap, AssocContainerId, AssocItemId, FunctionId,
     GenericDefId, HasModule, ImplId, Lookup, ModuleId, TraitId,
@@ -21,7 +25,7 @@ use crate::{
     primitive::{self, FloatTy, IntTy, UintTy},
     static_lifetime,
     utils::all_super_traits,
-    AdtId, Canonical, CanonicalVarKinds, DebruijnIndex, FnPointer, FnSig, ForeignDefId,
+    AdtId, BoundVar, Canonical, CanonicalVarKinds, DebruijnIndex, FnPointer, FnSig, ForeignDefId,
     InEnvironment, Interner, Scalar, Substitution, TraitEnvironment, TraitRefExt, Ty, TyBuilder,
     TyExt, TyKind, TypeWalk,
 };
@@ -757,20 +761,13 @@ pub(crate) fn inherent_impl_substs(
 /// This replaces any 'free' Bound vars in `s` (i.e. those with indices past
 /// num_vars_to_keep) by `TyKind::Unknown`.
 fn fallback_bound_vars(s: Substitution, num_vars_to_keep: usize) -> Substitution {
-    s.fold_binders(
-        &mut |ty, binders| {
-            if let TyKind::BoundVar(bound) = ty.kind(&Interner) {
-                if bound.index >= num_vars_to_keep && bound.debruijn >= binders {
-                    TyKind::Error.intern(&Interner)
-                } else {
-                    ty
-                }
-            } else {
-                ty
-            }
-        },
-        DebruijnIndex::INNERMOST,
-    )
+    crate::fold_free_vars(s, |bound, binders| {
+        if bound.index >= num_vars_to_keep && bound.debruijn == DebruijnIndex::INNERMOST {
+            TyKind::Error.intern(&Interner)
+        } else {
+            bound.shifted_in_from(binders).to_ty(&Interner)
+        }
+    })
 }
 
 fn transform_receiver_ty(